Triplet objective having a negative element formed of a copolymer of acrylonitrile and styrene



TRIPLET OBJECTIVE HAVING A NEGATIVE ELEMENT FOR OF A COPLYMER OFACRYLONITRILE AND STYRENE Filed Dec. 22, 1961 Fig.1

Fig. 2

SEARCH ROOM- 594,11 T2 mfg EQUIVALENT FOCAL LENGTH-100mm. f/ 8 LENS N VRA DI/ THICKNESSES I 1.492 57:4 R, 3l73mm. i',- 10. 18mm.

R 103.3 2.73 2 I. 564' 36.6 R3' 47J4 t;- 2.3,

R 4Q25 $z= 7.84- 3 1.492 524 R 35O.6 t 4.63

Fred .E'Allman r INVENTOR.

I y A Ami m United States Patent 3,194,116 TRIPLET OBJECTIVE HAVING ANEGATIVE ELE- MENT FORMED OF A COPOLYMER 0F ACRYLO- NITRILE AND STYRENEFred E. Altman, Rochester, N.Y., assignor to Eastman Kodak Company,Rochester, N.Y., a corporation of New Jersey Filed Dec. 22, 1961, Ser.No. 161,676 11 Claims. (Cl. 88-57) This application is acontinuation-in-part of my ap plication Serial No. 15,644, filed March17, 1960 now abandoned.

This invention relates to plastic lens elements.

The preferred embodiment thereof relates to photographic objectives ofthe type having three airspaced, simple elements, the outer ones ofwhich a e positive and the inner one of which is negative. Such a lensis usually called a Cooke triplet, and the type is widely used. It hasthe advantage of providing relatively good correction for aberrations ina simple construction using only three simple lens elements. Sincetriplets are relatively inexpensive but highly corrected, and since oneof the advantages of plastic lenses is the low cost of manufacture, andassembly, the present invention is particularly applicable to tripletlenses.

The object of my invention is to provide a plastic negative lens elementsuitable for use in compound lenses.

It is a particular object of the preferred embodiment of the inventionto provide such a plastic element which has the proper indices ofrefraction and dispersion to cooperate in a triplet with one or both ofthe two positive elements being of methylmethacrylate to give goodcorrection of color and all other lens aberrations.

An object of a particular embodiment of my invention is to provide ahighly corrected lens design which may be made up entirely of injectionmolded plastic elements with good mechanical and optical properties andwhich because of both the design and the use of injection moldedplastics, is relatively inexpensive to manufacture.

Such a lens was never made prior to the present invention. Theshortcomings of the prior efforts which were made will now be described.

In the case of glass lenses the choice of mechanically and physicallysatisfactory materials is wide, but the choice of plastics is relativelynarrow. Various methacrylates including cyclohexyl-methacrylate andparticularly methyl-mcthacrylate form excellent positive elements partlybecause injection molding thereof gives optical quality elements, butsuch plastics cannot be used in the negative elements of the same lenssystem since this would not permit achromatizing of the lens system. Ihave tried polystyrene as the negative lens with methylmethacrylate inthe positive lenses of a triplet, but the dispersive index ofpolystyrene is too low to give optimum correction of the otheraberrations when the lens is achromatic. When achromatization isobtained using styrene as the negative element, the overall length ofthe lens is too great to permit a reasonably wide field to be covered;if the lens is made shorter all aberrations including color becomeworse. I have also tried other plastics in the negative element. Benzylmethacrylate, mentioned in my US. Patent 2,430,550 and copolymers ofmethylmethacrylate with styrene have the desired intermediate V value(not as low as styrene), but both of these materials are unsuitable forinjection molding. The former must 3,194,116 Patented July 13, 1965 bepolymerized in the mold and so must the latter copolymer if striae areto be avoided. The net result of all such difiiculties has been theimpossibility of making a satisfactory (reasonably wide field) tripletor other compound len entirely of injection molded plastics, prior tothe present invention.

Thus, the primary object of the present invention is i to solve the ageold problem of making a high quality lens entirely of injection moldedplastic elements.

By the present invention it is possible to make a moderate aperture (f/8 lens of wide angular field and good correction or to make a highaperture (f/ 2.7) lens with satisfactory correction over a useful field.In fact it is possible, for example, to make, entirely with inexpensiveplastic elements, f/8 lenses covering 27 from the axis which are atleast equal in picture taking quality to lenses made with glass.

Various features which have been found useful in the design of glasslenses can be applied with equal effect when the lenses are made ofplastic. For example in some cameras it has been found desirable to gainadded improvement by curving the film surface slightly, in the longerpicture dimension, while employing a lens designed to have a curvedfield to match. This feature is employed in some embodiments of thepresent invention. Alternatively a field fiattener can be added insteadof curving the film plane; field flatteners are weak negative lensesimmediately in front of the image plane. Also compensation fortemperature changes may be provided by the mount described in patentapplication Serial No. 126,168 by C. Estes, filed July 24, 1961.

The use of plastic materials instead of glass in the manufacture ofphotographic objectives has many advantages. As mentioned above, someplastics may be injection molded to produce lens elements havingaccurately curved and highly polished surfaces and requiring a minimumof finishing operations. Such lens elements are of course much lessexpensive to produce than glass elements which must be ground andpolished and then finished individually. Furthermore, the need forselective assembly is reduced greatly or eliminated; this both reducesassembly costs and permits automated assembly.

Not all transparent plastic materials are suitable for use in inject-ionmolding of lenses. Some copolymer mixtures tend to becomenon-homogeneous during manufacturing operations, resulting in sucheffects as striae, variations in index and poor surface quality, in thefinished lenses. In the past, materials which are colored such as yellowor amber or which are slightly cloudy have not been considered suitablefor lens materials for obvious reasons. In such cases I have found thatthe expected detrimental effects do exist but are sometimes effectiveonly with positive lens elements. A satisfactory negative elementsaccording to the present invention is made of a material which is tooyellow in a positive lens. Any effective color in a camera lens is quiteobjectionable both because of the effect on color pictures and becauseof the added density. One known yellow material is a copolymer ofstyrene and acryonitrile having a published index of refraction of N=1.564. This material is available under various trade names, one ofthem be- I ing Bakelite C-ll, which contains about 72% styrene and 28%acrylonitrile. It is yellowish in appearance and when used in drinkingglasses, for example, -a blue dye of the same two materials, usuallywith higher percentages of acrylonitrile are well known as fibers andsheeting but are not generally molded into thick solids. They are notused in the present triplets since the higher V values are neitherrequired nor desired. It is well known that N changes at leastapproximately linearly with the percentage composition of either glassor plastic mixtures. My above mentioned experience and that of others,with copolymers of methylmethacrylate and styrene showed that V alsovaries in direct proportion to the percentage of each component. Thusany copolymer of acrylonitrile and styrene with a published N of 1.564will have the percentage composition required and will have a V valueabout 36.6. Increasing the acrylonitrile content lowers the N and raisesthe V.

According to the present invention and contrary to all priorconsiderations given to such materials, copolymers of acrylonitrile andstyrene are useful as a lens element, provided the lens element is anegative one. Negative lenses are thinner at the axis than at the marginwhich apparently reduces any adverse effect of the yellowish color andof any cloudiness. Clear and substantially or effectively colorlessnegative lens elements can be injection molded from copolymers ofacrylonitrile and styrene even though this material in bulk is yellow.Blue dye may be added if neutral coloris more essential than minimumdensity, but with or without the dye, such material is satisfactory as anegative lens element.

Furthermore I had the dispersive index V of the abovementioned copolymermeasured and found it to be about 37, one specific sample being 36.6 andanother being 36.0. This copolymer remains homogeneous during injectionmolding and gives a uniform finished product with optical qualitysurfaces. Thus the copolymer of acrylonitrile and styrene when injectionmolded was found for the first time to have (1) adequate uniformity foroptics, (2) the superior surface quality required by optics, (3) auseful dispersive index, one particular copolymer having exactly thedispersive index needed to cooperate with methylmethacrylate positiveelements and (4) no objectionable color or cloudiness in negativeelements even though it is yellow and slightly cloudy in bulk.

According to the preferred embodiment of my invention this copolymer isused as the negative elements of a triplet lens when amethylmethacrylate resin having an index of refraction N =l.492 and anindex of dispersion V=57.4 is used in one or both of the positiveelements.

The use of methylmethacrylate resin in optical lenses and prisms is notunknown but the present invention is the first time that theacrylonitrile styrene copolymer has been used in any lens element and itis useful provided the element is a negative one. Furthermore it is-particularly advantageous when a positive element in the same objectiveis made of a methacrylate. It is this combination which gives theunusually high correction in a Wide angle, moderate aperture, lens andwhich gives good correction in high aperture lenses. Specific examplesare given below. In the design of triplet lenses, when the dispersivelens has a higher index of refraction than that of the collectivelenses, the best corrections are obtained when the field is allowed tobe slightly curved. This curvature is adequately overcome by curving thefilm surface in the long dimension or horizontal dimension when a squareformat is used, as is commonly done for glass lenses or by using a fieldflattener, all as discussed above. Various examples of triplet lensesincorporating my invention are given below including Example 1 which hasan angular coverage which exceeds 33 from the axis with goodillumination and acceptable corrections in the edges of the field, withan aperture of f/8. Such an objective is shown in section in FIG. 1 anda table of the radii and spacings is given in FIG. 2. The table isrepeated below as Example 1, other examples at various apertures alsobeing given.

Example 1 [F= mm. [/8] Lens ND V Radii, mm. spacings,

R.=+ 31.7 l.=10.1 1 1.492 57.4 Rg=-l03.3

5z= 7.3 R5=+350. G 3* 1. 492 57. 4 l 4. 6

*Mcthylmethacrylate. copolymer acrylonitrile-styrene.

Example 2 [F =10O mm. 174.5]

Lens N V Radii, mm. spacings,

nun.

81=3. 5 Ra=- 34. 4 2 1. 564 36. 6 lg=2. 2

. Jz=8. 5 R5=+122. 4 3 1. 492 57.4 l =9. 2

Example 3 [F=100 mm. f/2. 7

Lens N V Radii, mm. spacings,

Ri=+35. 3 1 1. 492 57. 4 t1=28. 0

81= 6.7 Ru=-37. 7 2 1. 564 36.6 tg= 7.8

' 8z= 6. 7 R5=+55. 3 3 1. 492 57. 4 t =24. 2

The above Example 3 and also Examples 6 and 7 below are useful for 8 mm.motion picture cameras made up in A field flattener for use with Example4 is made of methylmethacrylate and has a radius of curvature on itsfront surface of 138.6 rnm., an axial thickness of 3.2 rnm., and a planorear surface adjacent to the image plane.

Some advantage is found in making a triplet lens with the front elementof methylmethacrylate and the negative element of the copolymeraccording to the invention, but using ordinary crown or barium crownglass in the rear positive element. The following are three examplescave, respectively, to the front. For a lens having a thereof: focallength other than 100 mm. all linear dimensions Example 5 may be scaledin the same ratio that the desired focal length bears to 100 mm. lF= mm.f/ l 5 Other copolymers of acrylonitrile and styrene can be used asnegative lens elements, since in accordance with Lens v Radil'mmg thepresent invention, the yellowish color becomes harmless, in sucharrangements. The particular copolymer Rl=+ 28,8 chosen in anyparticular lens will be one having the 1 1.492 57.4 t =9.5 desiredindices of refraction and dispersion to give the 81:4 2 correctionrequired. That is, such copolymers can be R3= 50.5 used in various formsof compound lenses. In triplets, 2 567 R4=+ the preferred copolymer isthe one herein described since s1=7.6 it has the indices required tocooperate with methylmetha- 3 L523 5&6 R5=+ Crylate. Furthermore theinvention is most useful with R 41.4 triplets since one of the primarypurposes of using injection molded plastic lenses is to reducemanufacturing E l 6 and assembly costs and among high quality compound[Fflmmm flz 7] lenses, the triplet is the least expensive and lendsitself best to the inexpensive assembly methods made available by theuse of plastic elements. Thus there is a peculiar Lens ND v iificooperation in a number of aspects when triplet lenses employ thisparticular copolymer in the negative element. R1=+37.2 Thus the tripletlenses and particularly the wide angle 1 492 R 8 tl=23-1 25 f/ 8 tripletlenses constitute the preferred embodiments of 2 .9 6,7 my invention,namely preferred embodiments of plastic 2 L 567 360 R3= 40-7 t2: 78lenses using this particular copolymer of acrylonitrile and R4=+30. 7styrene in the negative element.

R5=+565 The above eight examples all constitute species of the a 1. 52358.6 n=24.2 preferred embodiment of my invention although Exam- 2 pics5, 6 and 7 employ glass in the rear element.

It will be understood that variations and modifications ple 7 can beeffected within the spirit and scope of the inven- [F=1OO mm. f/2.7 tionas describedhereinabove and as defined in the ap pended claims. Lens NDV Radii, mm. spacings, I m:

1. A triplet lens comprising two positive elements of methylmethacrylateand, spaced and aligned between the 1 1. 492 57. 4 tl=23 7 positiveelements, a negative element formed of a copoly- R1=-123.9 mer ofacrylonitrile and styrene having an index of re- 4&8 fraction N about1.56 and an index of dispersion V 2.-. 1.567 36.0 n= 7.6 about 37.

R4=+ 82: 6 9 2. A lens covering a field of wide angle corrected for R5=+68.3 spherical aberration, coma, and chromatic aberration, 3 L611 53 4composed of three elements spaced along an axis and having their opticalaxes aligned, in which the two outer elements are positive and formed ofmethylmethacrylate 3 followmg lens is slmllarhtotExamp 1e fi 1s gplastic and the inner element is negative and formed of a g s ii g ig fs l 1.?Y: i f y copolymer of acrylonitrile and styrene having an indexof p refraction N of 1.56. Example 8 3. An objective havingapproximately the following [F=100 mm H8] characteristics:

Lens N V Radii, mm. spacings, Lens Nn V Radii Spaeings 0 1 1.492 57.4Rim!- n=7.s 1 1.49 57 t 1()f Rz=394.0 R7=1.0f

81 =5. 9 81=. 03f 2 1 567 as o Rab n=1s 2 1.56 37 R3= ,=,02

R5=+ 99 0 8z=9.3 R5 +3i5f 82=.08f o 3 1.49 57 t 3 1.49. 57.4 R= 40.8 is5.5 39/ s 05! In FIG. 2 and in the above examples, the lens elementsWhere f is h focal g h f the Objective, the numbers are numbered fromfront (long conjugate side) to rear, in the first column refer to lenselements numbered from N is the index of refraction of the lens materialfor front to rear, lens element number 2 being negative and the D lineof the spectrum, and V is the dispersive index formed f a copolymer ofacrylonitrile and styrene N is of the material. The radii of curvatureare given by R the index of refraction for the D line of the spectrum, Vwith the subscripts corresponding to lens surfaces numis the dispersiveindex, R to R are the radii of curvature bered from front to rear, whichis the direction that light of the refractive surfaces numbered fromfront to rear, travels through the lens when used in a camera. The andsigns referring respectively to surfaces convex thicknesses of the lenselements I and spacings s are simand concave to the front, t to t arethe thicknesses of the ilarly numbered from front to rear. The and signselements, and s and s are the airspaces between the eleattached to theradii refer to surfaces convex and conments.

4. An objective having approximately the following characteristics:

8 with an index of refraction N between 1.50 and 1.65 and an index ofdispersion about 59.

7. An objective having approximately the following characteristics: LensND V Radil Spacmgs Lens N D V Radii S pacings R1=+.35f 1 1.49 57 Rti=.23j

R .43 R 38 1.49 57 l I n=.241 O 1--. f 1.56 37 t2.08f :07

2 1. 57 as n=.0s R -+.55f R4=+ .37] 3 1. 49 57 73:24] 82:07; Re=-.38fR5=+ 3 1.61 59 !a=.25j

Rll=' .53] where f is the focal length of the objective, the numbers inthe first column refer to lens elements numbered from front to rear,lens element number 2 being negative and formed of a copolymer ofacrylonitrile and styrene N is the index of refraction for the D line ofthe spectrum, V is the dispersive index, R to R are the radii ofcurvature of the refractive surfaces numbered from front to rear, andsigns referring respectively to surfaces convex and concave to thefront, 1; to t are the thicknesses of the elements, and s and s are theairspaces between the elements.

5. An objective having approximately the following characteristics:

where f is the focal length of the objective, the numbers in the firstcolumn refer to lens elements numbered from front to rear, lens elementnumber 2 being negative and formed of a copolymer of acrylonitrile andstyrene N is the index of refraction for the D line of the spectrum, Vis the dispersive index, R to R are the radii of curvature of therefractive surfaces numbered from front to rear, and signs referringrespectively to surfaces convex and concave to the front, t, to 1 arethe thicknesses of the elements, and s and s are the airspaces betweenthe elements.

6. A triplet lens comprising, spaced and aligned, a front positiveelement of methylmethacrylate, a negative element of a copolymer ofacrylonitrile and styrene having an index of refraction N about 1.56 andan index of dispersion V about 37 and a rear positive element of glasswhere f is the focal length of the objective, the numbers in the firstcolumn refer to lens elements numbered from front to rear, lens elementnumber 2 being negative and formed of a copolymer of acrylonitrile andstyrene N is the index of refraction for the D line of the spectrum, Vis the dispersive index, R to R are the radii of curvature of therefractive surfaces numbered from front to rear, and signs referringrespectively to surfaces convex and concave to the front, t to 1 are thethicknesses of the elements, and s and s; are the airspaces between theelements.

8. A negative lens element formed of a copolymer of acrylonitrile andstyrene having an index of refraction N about 1.56 and an index ofdispersion V about 37.

9. A compound lens entirely of plastic in which a positive element isformed of methylmethacrylate and a negative element is formed of acopolymer of acrylonitrile and styrene having an index of refraction Nabout 1.56 and an index of dispersion V about 37.

10. A triplet lens comprising, spaced and aligned, a positive element ofmethylmethacrylate, a negative element of a copolymer of acrylonitrileand styrene having an index of refraction N about 1.56 and an index ofdispersion V about 37 and a positive element of glass with a refractionN between 1.50 and 1.65 and an index of dispersion about 59.

11. A compound lens in which a positive element formed ofmethylmethacrylate is axially aligned with, and air spaced from, anegative element formed of a copolymer of acrylonitrile and styrenehaving an index of refraction N about 1.56 and an index of dispersion Vabout 37.

References Cited by the Examiner UNITED STATES PATENTS 2,388,869 11/45Reiss 88-57 2,405,301 8/46 Grey 8857 2,423,491 7/47 Fairbank 88572,430,550 11/47 Altman et a1 8857 JEWELL H. PEDERSEN, Primary Examiner.

1. A TRIPLET LENS COMPRISING TWO POSITIVE ELEMENTS OF METHYLMETHACRYLATEAND, SPACED AND ALIGNED BETWEEN THE POSITIVE ELEMENTS, A NEGATIVEELEMENT FORMED OF A COPOLYMER OF ACRYLONITRILE AND STYRENE HAVING ANINDEX OF REFRACTION ND ABOUT 1.56 AND AN INDEX OF DISPERSION V ABOUT 37.